An automatic transmission is a gearbox that automatically changes gear ratios in response to vehicle and engine speed. Internal combustion engines work best at relatively high rotational velocities. An automatic transmission allows the internal combustion engine to be coupled to the wheels in a manner that allows a range of speed and torque outputs, while allowing the internal combustion engine to operate in its most efficient range.
One of the core challenges when automatic transmissions were first developed is what to do when the vehicle is at a stop (engine is moving, but wheels are not). Strictly mechanical solutions to attaching the engine to the transmission and the transmission to the wheels were attempted in the 1920s and 1930s. These solutions proved to be unduly complex and poorly functioning. As a result, automatic transmissions did not penetrate far into the car market.
The solution was found in fluid dynamics. A fluid coupling is a two element drive (impeller and turbine) that is incapable of multiplying torque. A torque converter has an additional element, called either a stator or reactor, that alters the fluid flow returning from the turbine to the impeller, allowing for a torque multiplication factor. This patent will use the term “reactor” for this element. In 1948, Buick introduced the Dynaflow transmission, which used the first mass-market torque converter.
Today, automatic transmissions rely on torque converters to transfer power from internal combustion engines to transmissions. Current worldwide estimates are that there will be 28.5 million automatic transmissions, using torque converters, produced in CY 2015, put in a total of 74.8 million light vehicles. Although less than 40% of the worldwide light vehicles use automatic transmissions with torque converters, in the U.S., the application rate is over 90%. There is a large market that is clearly not saturated, for both automatic transmissions and torque converters.
The Society of Automotive Engineers (“SAE”) specification J641 defines several quantities for a hydrodynamic drive. Speed Ratio is the output speed divided by the input speed (No/Ni). Slip is the difference between the input speed (Ni) and the output speed (No). The torque ratio is the output torque divided by the input torque (T0/Ti), also referred to as torque multiplication. The Capacity Factor, also called the K-Factor, is the input speed in rpm (Ni) divided by the square root of the input torque Ti. The K Factor varies with speed, meaning that there is a K Curve. The Coupling Range designates the range of operation at which the Torque Ratio is unity or near unity. The Coupling Point designates the point where the torque conversion range ends and the coupling range begins. A typical torque converter uses a hydraulic medium, an impeller, a turbine, and a reactor to transfer power from the engine to the transmission. The flywheel of the engine is mechanically connected to the torque converter cover, which in turn, is connected to the impeller assembly. The turbine is mechanically connected to the transmission via the input shaft. As the cover and impeller spin, the hydraulic fluid spins, causing the turbine to spin and drive the gearbox; the hydraulic fluid exiting the turbine flows into the reactor which in turn routes it back into the impeller. The reactor is mounted on the one-way clutch. A one-way clutch is a type of clutch that engages in one rotational direction, preventing reactor rotation, and freewheels (i.e., spins freely) in the other rotational direction, allowing reactor rotation.
In a typical torque converter, the output torque given to the input shaft is the sum of the input torque imparted by the impeller and a torque given by the reactor to the fluid. At lower Speed Ratios, the one-way clutch is engaged, and the reactor is lock-up (non-rotatable). When the Speed Ratio is relatively small and the torque converter fluid largely changes flowing direction, the reactor torque is positive, meaning that the output torque becomes larger than the input torque. As speed ratio increases, the torque on the reactor declines proportionally with declining torque ratio. When the Speed Ratio is at or above the Coupling Point, typically 0.8-0.85, the torque on the reactor reaches zero, the torque ratio becomes unity, and the one-way clutch disengages, allowing the reactor to free spin in the same direction as the turbine and impeller. If the brake of the vehicle is applied, the turbine is locked or restrained, allowing the wheels to come to a stop while the engine still rotates. This is the idling phase or mode. The idling phase lasts while the vehicle is at rest and, briefly, while acceleration starts. The stall phase has maximum torque multiplication. During the vehicle launch, the turbine is increasing in rotational velocity, but has a much lower magnitude than the impeller and its torque is higher than the impeller's torque. The torque converter connects an engine to the gearbox and multiplies engine torque for increased low-speed acceleration. In the current art, the reactor is non-rotatable during the stall and launch phase, and held non-rotatable by a one-way clutch.
In the current art of torque converters, a bypass clutch is used to mechanically connect, or gang, the turbine to the impeller as the velocity of the turbine approaches that of the impeller. A bypass clutch improves the efficiency of the torque converter. The bypass clutch may be either a slipping or a non-slipping clutch. To improve fuel economy, many current automatic transmissions will lock-up the torque converter with the bypass clutch significantly before the coupling point.
A lower K Factor means less slip. A higher K Factor means more slip. When the reactor blades spin freely, or when there is no reactor, the K Factor can be, typically, lower than when the reactor is non-rotating below the coupling point. During vehicle travel, the bypass clutch disengages and re-engages during several types of events. For example sudden throttle changes can cause the bypass clutch to disengage and re-engage. Additionally, the bypass clutch must typically disengage to enable gear changes during shifting. The bypass clutch re-engages after the shifting event. In these disengage and re-engage events, a lower K factor converter will slip less, consuming less fuel and causing less vehicle disturbance. During engine idling, a higher K Factor is desirable, for less engine load and less fuel consumption. Control of reactor rotation (or non-rotation) is important for shift quality and fuel economy, at idling, during launch and travel, and while shifting. A one-way clutch does not optimize the reactor rotation. As a result, unnecessary design trade-offs are made with respect to K-Factor, torque multiplication, shift quality, and fuel consumption.